In a human body, intracellular and extracellular osmotic pressures are kept constant and an action of kidney is properly maintained by an ion balance between sodium and potassium. However, in our eating habits of frequently using processed foods today, there is a tendency of an intake amount of sodium being large and an intake amount of potassium being reduced. Therefore, it is effective for management of a health condition to measure a ratio of Na/K.
A multi-ion sensor capable measuring concentrations of sodium ions and potassium ions in such as beverage is known as shown in, for example, Patent Literature 1. However, in the multi-ion sensor described in Patent Literature 1, after a concentration of sodium ions and a concentration of potassium ions are separately measured, a concentration ratio of sodium ions to potassium ions is obtained based on these measurement values.
In this multi-ion sensor configured to obtain a concentration ratio of sodium ions to potassium ions based on these measurement values after a concentration of sodium ions and a concentration of potassium ions are separately measured, it is necessary to present a constant reference potential to each of a sodium ion electrode and potassium ion electrode. Therefore, internal solution is required for a reference electrode. Therefore, it has been conventionally difficult to reduce a thickness of a multi-ion sensor integrally provided with a reference electrode to be thinner.